(a) Field of the Invention
Broadly speaking, this invention relates to telecommunications. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for automatically selecting the best one of n telecommunications channels.
(b) Discussion of the Prior Art
It is frequently necessary to establish a highly reliable telecommunications link over channels which are characterized by time-varying channel occupancy or uncertain communications support mechanisms. Heretofore, no really successful solution to this problem has been found except by the use of human operators located at both ends of the telecommunications link. However, in many instances, manning both ends of a communications link is undesirable and in some cases impossible, for example, when the far end of the link is in some inaccessible or dangerous location.
Related prior art apparatus include U.S. Pat. No. 3,160,813, issued Dec. 8, 1964, which utilizes a channel selection system wherein a transmitter and receiver sweep a frequency band in synchronism and the receiver registers the field intensity or signal-to-noise ratio to ascertain the frequency for which the received signal is the highest. The transmitter and receiver local oscillator then operate on that selected frequency. This requires synchronization between receiver and transmitter and prior communication to start the scans simultaneously and then relies on the strongest signal level to identify the best channel. Noise or interference can defeat this technique. Another system shown in U.S. Pat. No. 3,983,492, issued Sept. 28, 1976, also searches for the strongest received signal. A microprocessor and logic circuit, frequency synthesizer, tunable receiver and voltage comparator are used to determine the strongest signal.
The technique disclosed in the present application was originally conceived as a solution to the problem of achieving extreme reliability in a system which relied on both high frequency and very high frequency radio propagation to provide communications over difficult terrain. Since spectrum occupancy and propagation conditions on these frequency bands are extremely time varying, high reliability cannot be achieved by conventional single frequency communications. Of course, one skilled in the art will appreciate that the invention is not limited to radio communications but may be used with equal success on any transmission facility which is subject to varying transmission quality.
The technique disclosed herein provides a solution to the two main problems which are characteristic of the HF and VHF radio bands. Communications via HF skywave, necessary to achieve coverage over long distances or in mountainous terrain, is subject to propagation anomalies which are severely time varying and dependent on the time of day, the month, the season of the year and current sunspot activity. These variations prevent HF radio from providing reliable signal support on any one preselected frequency, since different frequencies are affected to varying degrees and at varying times by these phenomena. For example, frequencies which are suitable for night use are usually unsuitable during daylight, and vice versa. Communications at or above VHF are generally more stable in that they are subject to fewer propagation variations and, therefore, provide inherently more reliable communications. They are, however, subject to terrain masking. On the other hand, spectrum occupancy in both of these bands is severe and interference caused by noise and other users also prevents reliable links from being continuously available on any one frequency.
It is therefore acknowledged that a choice of alternate operating frequencies is not only desirable, but essential to highly reliable communication links at HF and VHF, as well as in other crowded portions of the spectrum. As discussed, since manned operation at both ends of the link is in many instances not only undesirable, but impossible, a means of automatically selecting satisfactory frequencies is required.
It is the intent of this disclosure to define a general system which is applicable to any communication medium which provides the user with a number of alternative channels for communication in an effort to automatically assure the availability of a high quality link. The discussion is generalized so as to apply equally well to HF and VHF, as well as to other frequency ranges or channels of interest.